This invention relates to a pressure swing adsorption process, and more particularly to a method of reducing the variability of the composition of a nonadsorbed product stream produced in a pressure wing adsorption process from a dead gas comprising a mixture of two or more gases.
When a gas mixture comprising two or more components having different adsorbabilities with a given adsorbent is processed in a pressure swing adsorption system using the given adsorbent and a conventional pressure swing adsorption cycle to produce an adsorbed product stream enriched in the most strongly adsorbed component and a nonadsorbed product stream that is depleted in the strongly adsorbed component and enriched in the remaining components, the composition of the nonadsorbed product stream generally varies somewhat over the course of the adsorption step of the process. This happens partly because of the particular step or steps that are performed in the pressurization of the adsorption bed and partly because of the difference in adsorbabilities of the various components of the feed stream.
When a pressure swing adsorption process is initially started up the adsorption bed is generally pressurized by cocurrently flowing fresh feed gas mixture into the bed. In subsequent cycles of the process the bed is usually repressurized in one or more steps, which generally include countercurrent pressurization of the bed with nonadsorbed product gas, or cocurrent pressurization of the bed with fresh feed mixture, or a combination of these steps. Each of these steps are consistent with the goal of ensuring that the nonadsorbed end of the bed is substantially free of the most strongly adsorbed component at the commencement of the adsorption step. When the bed is cocurrently pressurized with fresh feed, the most strongly adsorbed component of the mixture is concentrated in the feed end of the bed and the least strongly adsorbed component is concentrated in the nonadsorbed product end of the bed. During countercurrent pressurization with nonadsorbed product gas, any strongly adsorbed component remaining in the bed from the previous cycle is desorbed from the bed and forced toward the feed inlet end of the bed by the incoming nonadsorbed product gas, which is highly concentrated in least strongly adsorbed component.
During the adsorption step, feed gas mixture is passed cocurrently through the bed. As the feed mixture moves through the bed, a wave front is formed, behind which the adsorbate comprises most of the most strongly absorbed component. The gas stream passing through the wave front will, accordingly, be depleted of the most strongly adsorbed component. As the gas stream, now enriched in the less strongly adsorbed components of the gas stream, continues through the bed, a second wave front may be formed in the region beyond the first wave front if the remaining stream contains two or more less strongly adsorbed components having different adsorbabilities. The adsorbate in the region between the first and second wave fronts will contain most of the second most strongly adsorbed component, and the gas stream passing through the second wave front will be depleted in both the most strongly absorbed component and the second most strongly adsorbed component. This phenomenon is repeated until the gas stream is comprised essentially of a single gas or of two or more gases having the substantially equal adsorption rates.
As the adsorption step proceeds, all of the wave fronts will advance toward the nonadsorbed product end of the bed, and as they do so the more strongly adsorbed components will displace the adsorbed less strongly adsorbed components of the adsorbate and force them toward the nonadsorbed product end of the bed. Eventually the more strongly adsorbed components will break through the nonadsorbed product end of the bed and become part of the nonadsorbed product. Because of the above-described phenomenon, the nonadsorbed product stream leaving the bed, which was initially a high purity least strongly adsorbed component stream, will contain greater amounts of the more strongly adsorbed components as the adsorption step proceeds.
For most situations, the variability of the nonadsorbed product composition over the course of the adsorption step of a pressure swing adsorption process is of little consequence. However, in some cases, it is extremely important that the composition of this stream remain within certain limits throughout the adsorption step. For instance, when a gas mixture containing oxygen, a nonflammable gas and a flammable gas or oxygen and two flammable gases is subjected to a pressure swing adsorption process to recover the nonflammable gas or one of the two flammable gases, it is very important to ensure that none of the product streams leaving the adsorption bed constitutes a flammable gas mixture.
In this specification the term "nonflammable gas" means a gas that will not form a flammable mixture when mixed with oxygen in any proportions; the term "flammable gas" means a gas that when mixed with oxygen in certain proportions will form a flammable mixture; and the term "flammable mixture" designates a gas mixture containing a flammable gas and oxygen in such proportions that it will ignite or explode if heated to its self ignition temperature or if it comes in contact with a flame or a spark at the prevailing pressure.
As a specific example of a potentially hazardous situation, it may sometimes be desired or necessary to separate a given component from a gas mixture containing the given component, a flammable gas component and oxygen using an absorbent which most strongly absorbs the given component and least strongly adsorbs oxygen. Assuming that the concentration of the components in the gas mixture is such that the mixture is not a flammable mixture, the given component could be safely separated from the other components by a pressure swing adsorption process using the adsorbent, provided that the nonadsorbed gas product stream remains nonflammable at all times during the adsorption process. If a conventional pressure swing adsorption process in which repressurization of the adsorption bed is effected by bed equalization followed by nonadsorbed product backfill is used to separate the given component from the remaining components, the nonadsorbed gas product will comprise mostly oxygen at the initial part of the adsorption step. However, as the adsorption step proceeds, the concentration of flammable gas component in the nonadsorbed product stream gradually increases until finally a point is reached when the nonadsorbed gas product stream is about to become a flammable mixture. Because of this hazard it is not feasible to separate the given component from this gas mixture by such conventional pressures wing absorption techniques.
U.S. Pat. No. 4,498,910, issued to Benkmann on Feb. 12, 1985, discloses a method of recovering a hydrocarbon from a feed gas containing the hydrocarbon and a minor amount of oxygen by a pressure swing absorption process. The patentee purportedly avoids the development of a flammable gas mixture by cocurrently repressuring the adsorption bed with feed gas allowing the countercurrent desorption of the adsorbed component from the adsorbent.
It can be appreciated from the foregoing discussion that there is a need for a pressure swing adsorption process that can be safely and efficiently used to separate a multicomponent gas mixture into an adsorbed component product stream and a multicomponent nonadsorbed product stream wherein the tendency of the composition of the nonadsorbed product stream to vary over the course of the adsorbed step is minimized. The present invention satisfies this need.